The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for forming a film used for flattening a substrate surface having unevenness in a manufacturing process of a semiconductor device.
Formation of a very fine pattern and a multilayer interconnection structure are demanded according as high densification and high integration of a semiconductor device in recent years, and a tendency that the unevenness of the substrate surface becomes still more conspicuous is being developed.
However, as the unevenness of the substrate surface becomes more conspicuous, it becomes more difficult to form a highly precise fine pattern and disconnection of interconnection and the like are also liable to occur. Therefore, lowering of reliability of a device and lowering of yield of semiconductor devices are caused. Thus, techniques for flattening the substrate surface are required.
In the early stage of flattening techniques, a PSG film obtained by adding phosphorus to a SiO.sub.2 film has been used as a flattening film which is able to flatten a substrate surface at a comparatively low temperature. This PSG film has a nature that the flattening temperature is lowered by increasing an addition quantity of added phosphorus, but the film absorbs moisture and the film quality is deteriorated when the addition quantity of phosphorus is increased, thus becoming unfit for practical use. In a PSG film having phosphorus concentration showing no deterioration of the film quality, the softening point is approximately 1,000.degree. C. at the most. Thereupon, a BPSG film obtained by adding both boron and phosphorus having a softening point lower than that of the PSG film has been developed as a flattening film. For example, a flattening temperature at approximately 850.degree. C. has been obtained by adding boron and phosphorus into a SiO.sub.2 film at 3 to 4 mol %, respectively, and the foregoing is the main current of present techniques of flattening a VLSI as a low temperature process.
FIG. 12 shows diagrams showing a process of flattening a BPSG film by means of reflow related to a conventional example. In FIG. 12(a), a SiO.sub.2 film 23 is formed on a Si substrate 22, and a polysilicon film 24 formed in a pattern is formed on the SiO.sub.2 film 23. Next, a BPSG film 25 is formed by a Chemical Vapor Deposition (CVD) method as shown in FIG. 12(b).
Next, when heat treatment (reflow processing) is applied at 850.degree. C. for about one hour in nitrogen (N.sub.2) atmosphere, the BPSG film 25 is softened, melted and fluidized. Thus, the substrate surface is flattened. Further, the film quality of the heat treated BPSG film is made fine, thus forming a film of good quality having no hygroscopic property. When the substrate surface is flattened as described above, it becomes possible to make the pattern of interconnector or film formed thereon fine, thus enabling to manufacture a highly reliable semiconductor device having little disconnection and short-circuit of interconnection.
Hereupon, as refinement of a device is advanced, a low temperature, and consequently a still lower temperature of the flattening temperature (reflow temperature) becomes required, and it is demanded in recent years to obtain the flattening temperature at 800.degree. C. or below. With respect to a BPSG film (SiO.sub.2 -B.sub.2 O.sub.3 -P.sub.2 O.sub.5), the softening temperature can also be lowered by increasing the concentration of B(B.sub.2 O.sub.3) and P(P.sub.2 O.sub.5) in the film similarly to the case of the PSG film.
FIG. 8(b) shows experimental data showing the relationship between the total quantity of impurities (P.sub.2 O.sub.5 +B.sub.2 O.sub.3) contained in a BPSG film and a reflow angle .alpha. at the reflow temperature of 900.degree. C. (parameter is boron concentration), and a tendency that the reflow angle .alpha. becomes smaller as the total quantity of impurities (P.sub.2 O.sub.5 +B.sub.2 O.sub.3) is increased, i.e., a tendency of being flattened is shown as shown in FIG. 8(b). Besides, FIG. 8(a) is a diagram for explaining the definition of the reflow angle. However, as the impurity quantity increases, hygroscopic property of the film becomes higher, and phenomena such as precipitation (B.sub.2 O.sub.3 or boric acid, etc.), moisture absorption (generation of H.sub.3 PO.sub.4) and clouding on the surface (devitrification) become liable to be presented, which cause an obstacle in the next process. Namely, an unstable process is caused, and there are many problems in point of the reliability of the device, too.
FIG. 9 and FIG. 10 are diagrams for explaining the foregoing, and show the relationship between phosphorus concentration/boron concentration and film quality stability of the BPSG film. The axis of abscissa represents boron concentration (mol %), the axis of ordinate represents phosphorus concentration (mol %), and the characteristic curve shows a boundary whether the film quality is stable or not. Namely, the instability of the film quality is increased when going in the right upper direction with the curve as the boundary, and the film quality is stabilized more when going in the left under direction. The characteristic curve was determined by whether the film quality of the BPSG film was deteriorated or not within one week after the BPSG film was deposited. As described, the film quality is deteriorated in general as the phosphorus concentration/the boron concentration is increased. Besides, the substrate temperature when the BPSG film is deposited is used as the parameter in FIG. 9, and the ozone concentration when the BPSG film is deposited is used as the parameter in FIG. 10.
Further, FIG. 11 shows experimental data (parameters are boron concentration and phosphorus concentration) showing the relationship between the reflow temperature and the reflow angle .alpha., and shows a tendency of being flattened as the reflow temperature is increased as shown in FIG. 11 (F. S. Becker, D. Pawlik, H. Schafer and G. Standigl J. Vac. Technol. B4(3), 1986, pp. 732-744).
It has been in such a dilemma that, when the phosphorus concentration/the boron concentration is increased aiming at a low temperature, the film quality is deteriorated thereby to affect the device, and, when the phosphorus concentration/the boron concentration is decreased, reflow at a high temperature is compelled as described above. A limit is placed to 850.degree. C. at present for the reflow temperature of the BPSG film at concentration showing no deterioration of the film quality.
It is an object of the present invention to provide a method of manufacturing a semiconductor device which is able to lower a reflow temperature by utilizing a BPSG film or a PSG film having high impurity concentration as a flattening film.
Further, it is another object of the present invention to provide a method of manufacturing a semiconductor device which is able to arrange so that a BPSG film or a PSG film of high impurity concentration including a drawback that the film quality is deteriorated by moisture absorption does not absorb moisture.